This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to marking the substrates on which integrated circuits are formed so that the substrates can be associated with various pieces of information.
Monolithic integrated circuits are typically formed on substrates of semiconducting material, commonly called wafers. In general; wafers are fabricated by starting with a single seed crystal that is held at a specific crystallographic orientation and lowered into a melt of molten semiconducting material. For example, the semiconducting material may be one of silicon, germanium, a III-V compound such as gallium arsenide, or a mixture of such materials.
As the seed crystal is slowly withdrawn from the melt, material from the melt solidifies to it in the same crystallographic orientation as the seed crystal is held. By slowing withdrawing the seed crystal, a long cylindrical ingot of the semiconducting material, called a boule, is formed. Individual wafers are sliced from the boule at a desired angle to the axis of the boule. The wafers are then shaped to be circular and one side of the wafer is polished in anticipation for the integrated circuit processing to be performed thereon.
At some point in the wafer fabrication process, two modifications are made to the wafer. First, some type of feature is added to the wafer so that it can be repeatedly physically oriented to a known position at later points in time. For some wafers this feature is called a flat, where a portion of the peripheral circular curvature of the wafer is ground to a flat shape, and for other wafers this feature is called a notch, where a small circular portion is etched into the peripheral edge of the wafer.
The second type of modification is the addition of an identification to the wafer, typically called a wafer scribe. The wafer scribe usually consists of a string of alpha numeric characters that are etched into the top surface of the wafer, such as by laser ablation. The wafer scribe allows the wafer manufacturer to identify the wafer, such as with information as to the place of fabrication, the date of fabrication, and boule from which the wafer was cut. Typically, the final polishing of the wafer, as introduced above, is performed after the identification mark and orientation feature are formed.
Unfortunately, both the identification mark and the orientation feature tend to reduce the amount of surface area on the wafer that is available for the fabrication of integrated circuits. Because the identification mark is typically formed by recessing the surface of the wafer on which the integrated circuits are formed, the surface of the wafer in the region of the identification mark is rendered inadequate for the formation of integrated circuits. Further, the orientation feature typically removed a portion of the wafer, thus reducing the amount of surface area available for the formation of integrated circuits.
There have also been other problems reported with the use of traditional orientation features and identification marks. For example, notches need to be kept clean of processing materials, or they tend to contaminate the wafer or otherwise disrupt the desired processing of the integrated circuits. Identification marks must also be kept clean and free of circuitry, or they cannot be read and used to identify the wafer in the manner intended. Thus, traditional orientation features and identifications marks have drawbacks associated with their use.
What is needed, therefore, is a system for orienting or identifying wafers which overcomes at least some of the problems identified above.
The above and other needs are met by a method for marking a wafer that is cut from a boule. A surface of the boule is marked with an encoded marking that extends completely along a distance of the boule that is used for cutting wafers. The encoded marking is disposed substantially parallel to a length axis of the boule. The wafer is cut from the boule from within the distance, such that the encoded marking along the surface of the boule is disposed at a peripheral edge of the wafer. The encoded marking contains information in regard to the wafer.
In this manner, the boule is marked in a way that is easily accomplished, and which is retained in each wafer as it is cut from the boule. Further, the marking that is formed on the surface of the boule, and which is disposed at the peripheral edge of the wafer, can be used for physically orienting the wafer, and also for identifying the wafer. Thus, the encoded marking is able to function both as an identification mark and an orientation feature. Further, the encoded marking at the peripheral edge of the wafer does not significantly reduce the amount of surface area available on the wafer for the fabrication of integrated circuits. Further yet, the nature of the encoded marking reduces the degree to which the encoded marking impacts the processing of the integrated circuits on the wafer, such as from contamination or flow disruptions.
In various embodiments, there is an additional step of rounding the peripheral edge of the wafer in a manner that does not remove the encoded marking. The wafer is preferably formed of a semiconducting material, and most preferably the wafer is formed of at least one of silicon, germanium, and a III-V compound. The step of marking the surface of the boule preferably includes laser ablating substantially parallel lines in the surface of the boule. In another embodiment the step of marking the surface of the boule includes printing substantially parallel lines on the surface of the boule. The step of rounding the surface of the boule is accomplished prior to marking the surface of the boule in some embodiments. Preferably, the encoded marking comprises a bar code. The information preferably includes at least one of an index number, a wafer manufacturer identification, a date of manufacture, a location of manufacture, and a boule identification. Most preferably there is an additional step of adding a wafer identification to the encoded marking on the wafer, where the wafer identification is unique relative to the boule. Also described is a wafer marked by the method of claim 1.
According to another aspect of the invention there is described a method for identifying a wafer that is cut from a boule. A surface of the boule is marked with an encoded marking that extends completely along a distance of the boule that is used for cutting wafers. The encoded marking is disposed substantially parallel to a length axis of the boule. The wafer is cut from the boule from within the distance, such that the encoded marking along the surface of the boule is disposed at a peripheral edge of the wafer. The encoded marking contains information in regard to the wafer. The encoded marking on the peripheral edge of the wafer is read, and the wafer is thus identified.
In various embodiments the wafer is formed of a semiconducting material. Preferably, the step of marking the surface of the boule includes laser ablating substantially parallel lines in the surface of the boule. Most preferably, the encoded marking comprises a bar code. A wafer identification that is unique relative to the boule is preferably added after the step of cutting the wafer from the boule.
According to yet another aspect of the invention there is described a method for positioning a wafer that is cut from a boule in a known physical orientation. A surface of the boule is marked with an encoded marking that extends completely along a distance of the boule that is used for cutting wafers. The encoded marking is disposed substantially parallel to a length axis of the boule, and at a known location relative to a physical orientation of the boule. The wafer is cut from the boule from within the distance, such that the encoded marking along the surface of the boule is disposed at a peripheral edge of the wafer. The wafer is rotated, and the encoded marking on the peripheral edge of the wafer is detected as the wafer rotates. The rotation of the wafer is stopped when the encoded marking on the peripheral edge of the wafer is disposed in a desired position.
In various embodiments, the step of marking the surface of the boule includes laser ablating substantially parallel lines in the surface of the boule. Preferably, the encoded marking comprises a bar code. A wafer identification that is unique relative to the boule is preferably added after the step of cutting the wafer from the boule.